1. Field of the Invention
The present invention relates generally to illuminated keyboard switches and displays and more particularly to such switches and displays illuminated by means of light emitting diodes (LEDs).
2. Discussion of Prior Art
With the proliferation of portable cellular telephones and other keypads and/or keyboards having backlit keys and displays, there is a need to provide a controlled distribution of light across the surface of the key or display.
Keypad switches, often times referred to as multi-layer backlit membrane switches such as those discussed and described in U.S. application Ser. No. 07/669,958 filed Mar. 15, 1991 entitled "BACKLIT TACTILE KEYBOARD WITH IMPROVED TACTILE AND ELECTRICAL CHARACTERISTICS", are well known in the art.
FIG. 1 of the present application illustrates one embodiment of a membrane switch disclosed in the '958 application (it is identified as FIG. 4) and while there are numerous other modifications and variations on membrane switches discussed in other applications and prior art disclosures, the problem with respect to illuminating such backlit switches and computer displays remains with all such devices.
The prior art membrane switch disclosed in FIG. 1 comprises a number of individual elements. A rubber keypad cover 10 is provided which has raised keys 12 located thereon. In a preferred embodiment, the keypad cover is bonded to an actuator spacer 14 by an adhesive layer 16. The actuator spacer is also bonded to a conductive dome membrane which in turn is mounted to spacer 20 with additional adhesive 16. Spacer 20 is in turn mounted to printed circuit board 22 by a further adhesive layer 16.
As shown in FIG. 1, the printed circuit board has a plurality of apertures 24 through which extends light emitting diode 26. The conductive dome membrane includes a large radius dome 28 and a small radius dome 30. Conductive traces 32 on the printed circuit board comprise the elements of the switch which are shorted by conductive material, 34 located on the lower surface of the large radius dome. The large radius dome serves to short the conductive traces 32 when raised key 12 is depressed. The large radius dome inverts and the conductive material 34 physically bridges across the conductive traces 32.
A further embodiment of a membrane switch is also illustrated in U.S. Ser. No. 07/751,185 filed Aug. 29, 1991 which is a continuation-in-part from the previously mentioned '958 application. In the continuation-in-part, a membrane switch is illustrated which does not rely upon the bridging of conductive traces 32 but rather has one conductor electrically connected to the printed circuit board and located on the lower surface of the conductive dome or alternatively in a separate shorting layer with the other conductor located on or adjacent the printed circuit board.
In either the '958 and '185 applications as well as in other known backlit membrane switch embodiments, the light emitting diode 26 provides a high level of illumination for its size. However, directly above the bulb, as would be expected, the light intensity is greatest but as one moves along the lower surface of the cover 10 away from the point directly over the light emitting diode, the intensity decreases dramatically until the edge of the spacer or other opaque material is reached, at which point the light intensity decreases to zero.
FIG. 2 illustrates the variation in light intensity (on the vertical scale) with respect to position where the position indicated corresponds to the position related to the light emitting diode in FIG. 1. Often times the maker of membrane switches will provide a cover which is not transparent but merely translucent which, as a result, tends to diffuse the light intensity somewhat. In fact, in the embodiment of FIG. 1, if a diffusing rubber were utilized as the keyboard cover, the consequence of having the raised key 12 would be that there would be much greater diffusion directly over the LED and less diffusion elsewhere. However, even in this circumstance, there would be a "hot spot" or a significant change in intensity of the LED as perceived by an operator using such a key.
Reflective Mylar films having half tone patterns thereon are known for use in combination with fluorescent lamps to provide a relatively even light distribution for backlit inspection tables. The Mylar film has a pattern of metalization which reflects a greater amount of light in the vicinity of the fluorescent light bulb and a lesser amount of light a distance away from the light bulb resulting in a more even distribution of light transmitted therethrough.
While the keypad/membrane switch industry has obviously flourished without solving the problem of intensity variations along the keys. However, many times a key includes indicia (alpha numeric characters or other keyboard symbols) printed on or molded into the key. If the key were more uniformly illuminated, the visual perception of such indicia by the operator would be improved. Computer keypads, keyboards and other elements are also using to a greater degree display areas which are illuminated by one or more light emitting diodes. The same problems exist in that, over the display area, there will be a wide variation in illumination intensity caused by the "hot spots" associated with the individual light emitting diodes utilized.
A further problem occurs in many prior art keypads in that where some keys are illuminated and other keys aren't, there is a sufficient amount of light bleed through or "crosstalk" between keys that it is difficult to determine which key is actually illuminated and which key is not. Similarly, where keys are illuminated with different colors, the "crosstalk" between adjacent keys serves to confuse the color coding.